The overall goal of this proposal is to combine the principles of time reversed acoustics (TRA) with MR detection of shear wave propagation in order to create a robust and quantitative assessment of tissue elasticity. As is well known, the mechanical properties of tissue are often important surrogate measures of disease burden. To address these concerns, recent advances in Magnetic Resonance Elastography (MRE) have shown promise in visualizing breast cancers and other tissue pathologies. The research proposed here will develop new methods to measure tissue elasticity of acoustically hidden organs such as brain or liver. To achieve this overall goal we will: 1) Conduct laboratory studies on heterogeneous tissue mimicking phantoms to evaluate technical requirements to MR Shear Wave Elasticity Imaging (SWEI) system and define specification for the TRA focusing system. 2) Construct an MR compatible TRA system for highly localized remote generation of radiation force in tissue phantoms. 3) Integrate the MR compatible TRA system with the MRI system and demonstrate the principle and the feasibility of the MR SWEI system based on TRA.